The IAU has proposed a definition which would add hundreds of new planets to our solar system!

From the time of the announcement of the discovery of 2003 UB313 in late July 2005, the "planetary" status of 2003 UB313 and of Pluto have been in limbo. The International Astronomical Union (IAU), the group charged with classifying objects in space, has just released a proposed definition and will hold a vote on this proposal on August 24th. If you want to read about the IAU proposal, immediately jump to 4 below!

Why is there a problem with Pluto?
What are the possible solutions to the Pluto problem?
    1. Demote Pluto (8 planets)
    2. Keep the status quo (9 planets)
    3. Let in the newcomer (10 planets)
    4. Leave no ice ball behind (53 planets) <-- this is the IAU recommendation!
    5. Do nothing (how many planets then????)

The IAU proposal officially recognizes only 12 planets; where does the number 53 come from?
How many planets are there in the Kuiper belt?

What should the public think about 53 planets?

How am I going to vote on the proposal?

A little background: Why is there a problem with Pluto (or 2003 UB313)?

Pluto and 2003 UB313 are significantly smaller than the other planets. If you were to start to classify things in the solar system from scratch, with no preconceived notions about which things belong in which categories, you would likely come to only one conclusion. The four giant planets -- Jupiter, Saturn, Uranus, Neptune -- belong in one category, the four terrestrial planets -- Mercury, Venus, Earth, Mars -- belong in one category, and everything else belongs in one or maybe more categories. You wouldn't lump the largest asteroid -- Ceres -- in with the planets, you would group it with the other asteroids. Likewise you wouldn't group the largest object in the vast swarm of objects beyond Neptune (the "Kuiper belt") with anything other than the Kuiper belt. The current word "planet" encompasses the group of giant planets and the group of terrestrial planets and the awkwardly ventures out into the Kuiper belt to take in one or two of the largest of those objects. Using the word in this way makes no scientific sense whatsoever, hence, the issue with Pluto.

What are the possible solutions to the Pluto (and 2003 UB313) problems?

I. Demote Pluto and 2003 UB313  (8 planets)

The simplest solution is the one that makes the most people cringe: admit that we made a mistake in 1930 by calling Pluto a planet. We have eight planets, and many thousands of asteroids and many thousands more Kuiper belt objects. Pluto is simply the second largest of the known Kuiper belt objects (2003 UB313 is the largest). Ceres is the largest asteroid. Case closed.

But we can't demote Pluto, can we? Well, of course we can. We did the same thing to the asteroid Ceres more than 150 years ago. When it was first discovered in 1801 it was declared to be a planet (the 8th, actually). Then another asteroid was found. Another planet! Then another. Planet again! This got old quickly and soon these tiny bodies were realized to be part of a vast population of rocks orbiting between Mars and Jupiter. They were wisely grouped into a single category, the word "asteroid" was born, and Neptune was the new 8th planet.

It takes guts to demote a planet that many people claim to love.  But if the IAU had made this decision and stuck to it it would only take a generation for everyone to accept the idea. People would even learn that science is capable of correcting itself when it makes errors, which is a useful lesson to see in action.

As the discoverer of 2003 UB313, would I be upset by this decision? No. Scientific decisions should be based on science, not sentiment. It would be an excellent choice. I'd be sad to miss the chance to have discovered the 10th planet, but I'd get over it.

Why didn't the IAU propose this definition? I think that astronomers are as sentimental as the rest of the world and couldn't stomach removing Pluto. Probably they also couldn't stomach the criticism that would follow.

II. Keep the status quo (9 planets)

We could always just say "OK, the word planet is an historical word, not a scientific word, so let's just leave it at nine and ignore anything else that comes in." I don't actually think anyone ever seriously considered this idea, though it is sometimes floated around. This definition would have the unintended effect of sending the signal that discovery in the solar system is complete. It doesn't matter what else we might find, we already know all the planets there are to know. This signal would be a very very bad signal. To my knowledge no one has seriously proposed this idea.

III. Let in the newcomer (10 planets)

A very simple solution to the Pluto/UB313 problem is to just define Pluto to be a planet (much like in II. above) and say that anything larger (currently only 2003 UB313) is also a planet.

Why do this? It makes no scientific sense at all, yet it appears to be what most people on the street think that the definition of "planet" should be. What are astronomers to do in a case where the public clearly thinks one thing and scientists another?

One interesting question to ask is "who is most affected by this decision?" Will it affect astronomers? Not at all. If there are officially 8 or 9 or 10 or 53 planets astronomers will continue the business of science by studying these objects to figure out what they are made of, how they were formed, and what they can tell us about the history of the solar system. In scientific discussions the word planet is hardly ever used.

Will it affect the public? Much more, it seems. The only reason that astronomers are spending so much time and effort on this question is because of the effect that it appears that it will have on the public.

Given that it is the public, rather than astronomers, who care and will be affected, one suggestion is that the astronomers simply get over it. To most people the word "planet" is more cultural than scientific. It is part of the mental landscape that we use to organize our ideas of the universe around us. The best analogy I can come up with is with the word "continent." The word sound like it should have some scientific definition, but clearly there is no way to construct a definition that somehow gets the 7 things we call continents to be singled out. Why is Europe called a separate continent? Only because of culture. You will never hear geologists engaged in a debate about the meaning of the word "continent" though. When geologists talk about the earth and its land masses they define precisely what they are talking about; they say "continental crust" or "continental drift" or "continental plates" but almost never "continent." 

Astronomers might be wise to learn from the geologists. Let culture define "planet" and let astronomers get back to the more important business of actually doing science.

IV. Leave no ice ball behind (53 planets, and counting)

This definition is the one the IAU chose to propose

There is a fourth solution, which is to fix the problem of Pluto being too small by making many many more planets so that Pluto is no longer even close to the limit. Even better, make the new definition have a scientific basis.

Such a solution would be to declare that everything that is large enough to be round due to its own self-gravity is a planet. As long as it is not a moon.

Why round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. By this proposed definition, you would then have a planet.

All of the ten current planets -- including Pluto and 2003 UB313 -- are round, so this means we don't have to demote anybody. What else is large enough to be round? The asteroid Ceres -- that one that was once a planet and got demoted -- is, in fact, round, so more than a century after its demotion it would be back to being a planet.

While the IAU is only officially willing to call 12 objects in the solar system round, we know with very little doubt that the Kuiper belt is home to perhaps a hundred or more round objects. We don't know the precise number because we don't know exactly how big an icy object in the Kuiper belt has to be to be round, but if we look at the icy satellites of the giant planets we see that everything larger than 400 km (250 miles) across is round while things smaller than 200 km (125 miles) across are not round. So somewhere in between is the transition. In the Kuiper belt we currently know of about  44 objects (including Pluto and 2003 UB313) that are larger than 400 km, so, at a minimum, we have 44+8+1=53 planets, by this scheme. We are not through searching the Kuiper belt, but when we are we are likely to have about 100 planets. 

Most people might think that a proposal to suddenly go from 9 to 53 planets would have no chance of passing, but I give this one good odds of passing the IAU vote. Why? It sounds scientific, it saves Pluto, and it suddenly makes many more people discovers of planets. Of course, it does even greater damage to the popular concept of the word planet by suddenly adding 44 new ones, all of which are so small that they could easily fit all together inside the earth's moon (which, of course, doesn't count as a planet) with plenty of room to spare, but perhaps that's a small price to finally have a definition after all of this time.

V. Make no decision

While the IAU has proposed a definition, the membership may vote no. What happens then? We are left in the same state we started, with no scientific definition, but with most people thinking there are 9 or 10 planets. I think this result is essentially similar to II. above. I give it a 30% chance.

The IAU proposal officially recognizes only 12 planets; where does the number 53 come from?

By the proposed IAU definition, anything large enough to be pulled by its own gravity into the shape of a sphere and which is in orbit around a star is a planet. The proposal officially recognizes 12 planets (the nine previously recognized plus Ceres and Pluto's moon Charon plus 2003 UB313) creates a complex committee procedure for an object to become officially recognized. This part of the proposal is perhaps the weakest. In no other area of astronomy is there a definition for a class of objects and then a committee that has to decide if an object fits the definition. There are simply definitions. If an object fits the definition it is part of the class. If the IAU proposal is accepted then scientifically all of the spherical  objects out there are indeed classified as planets, regardless of  how long it takes for a committee to officiailly declare them to be so.

A relatively simple analysis show that there are currently 53 known objects in the solar system which are likely round. Another few hundred will likely be discovered in the relatively near future. Regardless of what the official count is from the IAU proposal these object all fit the scientific definition of the word planet and if the scientific definition is to have any credibility they should all generally be considered planets.

What should the public think about 53 planets?

Most people, when first confronted with a proposal to make 44 new planets in the solar system, seem to react by looking blankly for a second, then shaking their heads and muttering something about astronomers being crazy. Astronomers are not actually crazy, at least most of them. Astronomers have needed a good scientific definition of the word "planet" for many years now and this one works well for scientists. It doesn't, however, work terribly well for the rest of the world. The solution is the one that should have happened long ago: a divorce of the scientific term "planet" for the cultural term "planet." No one expects school children to name the 53 planets (most, in fact, don't even have names). If I were a school teacher I would teach 8, or 9, or perhaps 10 planets and then say "scientists consider many more things to be planets too" and use that opportunity to talk about how much more there is in the solar system. But at the end of the day I would talk about 8 or 9 or 10. Not 53.

Culture and science have always meant something different when they use the word planet, and with this new scientific definition so clearly far removed from what the rest of the world things a planet is there will no longer be any need to confuse the scientific word with the cultural one.

How am I going to vote on the IAU resolution?

This one is easy to answer. I am not an IAU member, I took no part in drafting the resolution, and I get no vote. If I were to vote, however, I would have to decide that while the definition itself is viable the extra non-scientific beauracratic barrage attached to the resolution would doom it for me.